Such ultrasonic transducers or ultrasonic flow meters are used in the intake tract and/or the exhaust tract of internal combustion engines, especially for the volume flow or mass flow measurement of air. In those cases, ultrasonic transducers are typically used which are both able to emit ultrasonic waves into a fluid medium, that is, a gas and/or a fluid, and are able to receive ultrasonic waves. Ultrasonic signals are usually transmitted through the flowing fluid medium from an emitter to a receiver, and in the process, the transit time, transit time differences or phases or phase differences of the ultrasonic signals, or even the combinations of these measured variables, are measured. These signals are influenced by the flow of the fluid medium. It is possible to infer the flow speed of the fluid from the degree of the influencing of the transit time. Examples of such ultrasonic transducers, which are also able to be modified, are represented in DE 42 30 773 C1, in EP 0 766 071 A1 or in DE 10 2007 010 500 A1.
In ultrasonic transducers or ultrasonic flow meters, for instance, of the type described above, especially for ultrasonic transit time measurement and/or ultrasonic flow measurement, particularly in gaseous fluid media such as air, so-called matching elements are typically used, for instance, in the form of one or more matching layers. These matching elements take into account the fact that the oscillation energy of injection into the fluid medium that is generated by the usual ultrasonic generators, such as piezoceramics, has to overcome a large acoustical impedance difference, for instance, by a factor of 6×10−5. Due to this, as a rule, approximately 99.9995% of the sound energy on the way from a piezoceramic into air is reflected back at the corresponding boundary surface, and is not usable for the measurement. The same reflection loss occurs again at the second receiving transducer element, which may also be identical to the first transducer element. In order to improve the acoustical coupling between the piezo element and the fluid to be measured, as a rule, matching elements, for example, having one or more matching layers are used, particularly in order to increase the signal level swing of ultrasonic flow meters. These matching elements are used for impedance matching and have acoustical impedances which lie between those of the piezoelectric transducer elements and the fluid medium. Such matching layers may include, for instance, λ/4 layers as diaphragms. For instance, ultrasonic transducers are familiar which have sound-radiating resonance elements or matching elements, such as metallic diaphragms or a λ/4-impedance-matching layer. Such ultrasonic transducers may be used particularly for flow measurement on air, an air mass signal within a systems control of an internal combustion engine being able to be derived from this flow measurement, for example.
One problem with known ultrasonic transducers or ultrasonic flow meters, however, is that they have to meet the requirements of particular pressure demands. A pressure-resistant ultrasonic transducer design is needed for this purpose. Ultrasonic flow meters are used, for example, for measuring flow in internal combustion engines and particularly in the motor vehicle after a turbocharger and/or a charge air cooler. This position has application advantages and advantages with respect to the system control of the internal combustion engine. However, in such installation positions, the maximum pressures, to which the ultrasonic transducers are exposed, are typically 2 bar to 6 bar. Other applications are also possible.
In ultrasonic transducers or ultrasonic flow meters, which have to meet such high requirements with respect to pressure tightness, the sound-radiating surface is usually designed as an integral component of the transducer housing or the flow tube. With that, however, this sound-radiating surface is usually connected to these components, such as the transducer housing and/or the flow tube, in a hard manner, so that great coupling exists with respect to the spreading of structure-born noise. Such a poor decoupling between the ultrasonic transducer and the surrounding housing or a surrounding flow tube is disadvantageous, however, for the signal quality of the ultrasonic transducers. In order, nevertheless, to provide a certain decoupling, elastomers are frequently used, such as silicone molded parts and/or O-rings. These, in turn, however, reduce the coupling efficiency of the ultrasonic signals into the fluid medium and the coupling-out efficiency of ultrasonic signals from the fluid medium into the transducer. There exists, basically, a conflict of aims between the requirements on the pressure resistance and the required structure-born sound decoupling, since the materials required for damping the relatively hard piezoceramics for sufficient energy coupling have to have a high acoustical impedance, but with that, they don't decouple sufficiently with respect to housing materials that are also hard. On the other hand, additionally applied materials, which would be suitable for decoupling, are too soft to hold the transducer core in a stable position, in response to a pressure exerted frontally from the side.